If you want to make an offset of multiple linear entities you can use 'Offset Multiple to Inside'. After you select the entities you want you can set the distance in the main menu. Besides the distance you can also select a layer on which the offset should be placed. By default the command will place the offset on the same layer as the source entity. If you don't need the source entities anymore you can let the command erase the source entities right away. The command will make an offset to the inside if possible. If the offset cannot be made the command will let you know how many offsets were not able to be created.